(1) Field of the Invention
The present invention relates to a polyester fiber suitable for use mainly in the production of industrial materials such as tire cords, V-belts, conveyor belts and hoses, and to a process for the preparation of this polyester fiber. More particularly, the present invention relates to a polyester fiber having an excellent dimensional stability, an enhanced toughness, and a latent high-tenacity performance, i.e., a final treated and processed product of which, for example, a treated cord or a cured cord to be used as a reinforcer for a rubber structure, has a high tenacity, a low shrinkage, a high modulus and a high chemical stability and therefore is useful as industrial materials, and to a process for the preparation of this polyester fiber.
(2) Description of the Related Art
A polyester fiber, especially a polyethylene terephthalate fiber, has well balanced and high tenacity, modulus and dimensional stability (low shrinkage), and is widely used as a reinforcer for a rubber structure such as a tire, a V-belt or a conveyor belt. Recently, the field of application of the polyester fiber has been broadened, and to be able to use the polyester fiber as a reinforcer instead of the "rayon" used as a carcass material of a radial tire and as a substitute for "Vinylon" used in the field of industrial materials, the polyester fiber must have a higher modulus, a lower shrinkage and a higher fatigue resistance. Processes for the preparation of polyethylene terephthalate fibers excellent in these characteristics are disclosed, for example, in Japanese Unexamined Patent Publication No, 53-58031, Japanese Unexamined Patent Publication No. 57-154410, Japanese Unexamined Patent Publication No. 57-154411, Japanese Unexamined Patent Publication No. 57-161119, Japanese Unexamined Patent Publication No. 58-46117, Japanese Unexamined Patent Publication No. 58-115117, Japanese Unexamined Patent Publication No. 58-186607, Japanese Unexamined Patent Publication No. 58-23914 and Japanese Unexamined Patent Publication No. 58-116414.
According to these known processes, polyethylene terephthalate is melt-spun, the as-spun filament yarn is taken up at a relatively high spinning speed of 1,000 to 3,000 m/min under a high tension to obtain a highly oriented undrawn filament yarn having a birefringence of 0.02 to 0.07, that is, POY, and this POY is heat-drawn at a low draw ratio of 1.5 to 3.5.
The polyester fibers according to the processes as described above (hereinafter referred to as "POY/DY") have high modulus and low shrinkage as compared with the conventional high-tenacity fiber, that is, a high-tenacity fiber (hereinafter referred to as "UY/DY") obtained by taking up a melt-spun filament yarn at a low spinning speed of less than 1,000 m/min under a low tension to obtain a lowly oriented undrawn filament yarn having a birefringence not larger than 0.01 and heat-drawing the lowly oriented undrawn filament yarn at a high draw ratio of 4 to 7. For example, if this polyester fiber is used as a carcass material of a radial tire, tire performances such as the driving stability at a high speed and the comfort when driving are improved and the percentage of defective tires is reduced, and therefore, the polyester fiber makes a great contribution to an improvement of the productivity.
Nevertheless, the polyester POY/DY having such excellent characteristics has some problems as described below. First, the tenacity and elongation at break are obviously lower than those of polyester UY/DY. The present inventors found that if the elongation at break of the fiber is low, the tenacity is extremely reduced during the twisting step or the dipping treatment and the cord made therefrom has an undesirably low tenacity, and that if the tenacity of the fiber is low, when the fiber is used as a reinforcer for a rubber structure such as a tire or a V-belt, the fatigue resistance is low and this low fatigue resistance causes a serious practical problem. If the amount of the reinforcing fiber is increased to obtain a high tenacity of the rubber structure, the cost is increased and the high-speed performance is reduced by the increase in weight. This is serious particularly in the case of a large tire.
The polyester filament yarn proposed in Japanese Unexamined Patent Publication No. 53-58031 has a relatively high tenacity of 7.3 to 9.1 g/d as disclosed in the examples of this patent publication, but since the elongation at break is very low, i.e., 6.7 to 8.3%, the tenacity is greatly reduced during the twisting step and the reduction of the tenacity is extreme upon application of an adhesive, and when subjected to the heat setting treatment and dipping treatment. Accordingly, the tenacity of the obtained treated cord is lower than 6 g/d, and to be able to use this cord as a reinforcing cord for a rubber structure, a further improvement of the tenacity is required.
In the process for the preparation of this polyester filament yarn, the as-spun filament yarn is quenched in a gas atmosphere maintained at a temperature lower than 85.degree. C. just below the spinneret under a condition wherein the spinning speed is relatively high. A known method of drawing industrial polyester filament yarns is adopted for the drawing, and therefore, to increase the modulus of the drawn filament yarn, the POY is drawn until almost broken, and a problem of frequent yarn breakages or filament breakage arises.
In Japanese Unexamined Patent Publication No. 57-154410 and Japanese Unexamined Patent Publication No. 57-154111, as the means for solving the foregoing problems, the applicant proposed the process in which a high-temperature atmosphere is maintained just below the spinneret and the terminal modulus of the obtained polyester filament yarn (hereinafter referred to as "raw yarn") is controlled to a level lower than 15 g/d.
In the process disclosed in Japanese Unexamined Patent Publication No. 57-161119 and Japanese Unexamined Patent Publication No. 58-46117, the toughness of the raw yarn and cord made therefrom is considerably increased, but the tenacity of the treated cord is 6.6 g/d at highest.
When the draw ratio is merely increased to obtain a high tenacity of the raw yarn, the elongation at break of the obtained high-tenacity raw yarn becomes lower than 10%, and when a greige cord is formed by twisting the raw yarn and a treated cord is obtained by subjecting the greige cord to the dipping treatment, a special means is not adopted for moderating the reduction of the tenacity, and hence, it is impossible to obtain a product in which the requirements of high tenacity and high fatigue resistance are both satisfied.
In the process proposed in Japanese Unexamined Patent Publication No. 58-115117, it is intended to increase the tenacity of the raw yarn and cord made therefrom by heat-drawing POY composed of a polyester having a high degree of polymerization. However, since a high dimensional stability must be simultaneously obtained, the level of the tenacity in the obtained treated cord is inevitably lower than that in conventional UY/DY.
In the process proposed in Japanese Unexamined Patent Publication No. 59-116414, since the heat drawing is carried out at a relatively low temperature, the drawing tension is increased and the maximum permissible draw ratio is reduced. Further, since a condition resulting in a low relax ratio is adopted, a raw yarn having a high tenacity and a high elongation at breakage cannot be obtained. Furthermore, the tenacity retention ration is very low and the tenacity is about 6.3 g/d which is approximately the same level as that of conventional POY/DY.